Auxins are a major class of plant hormones that influence diverse aspects of plant behavior and development including vascular tissue differentiation, apical development, tropic responses, and organ (e.g., flower, leaf) development. The term “auxin” refers to a diverse group of natural and synthetic chemical substances that are able to stimulate elongation growth in coleoptiles and many stems. Indole-3-acetic acid (IAA) is the principal auxin in higher plants, though other molecules such as 4-chloroindole-3-acetic acid and phenylacetic acid have been shown to have auxin activity. Synthetic auxins include 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4-dichlorophenoxyacetic acid (2,4-D); both are commonly used as herbicides.
Distribution of auxins in concentration gradients within plant organs enables auxins to convey to cells their relative location, allowing the plants to respond accordingly to a given stimulus. A classic example that illustrates auxin action is the differential growth and curvature of etiolated coleoptiles exposed to light. It is believed that the phototropic stimulus results in a lateral redistribution of auxin in the coleoptile such that the shaded side has a higher auxin concentration than the illuminated side. With more auxin stimulating cell elongation on the shaded side, the end-result is the apparent bending of the coleoptile towards the light source.
The foregoing description underscores the importance of polar transport in auxin function. Not surprisingly, a number of genetic and physiological studies have focused on the polar auxin transport system operating in plant cells. Arabidopsis mutants with impaired auxin transport capabilities exhibit varying phenotypes: pin1 mutants develop naked, pin-like inflorescences with few normal flowers (Gälweiler, L. et al., (1998) Science 282:2226-2230), while defects in pin2 (also called eir1 and agr1) are restricted to the root, altering growth and gravitropic response (Luschnig, C. et al., (1998) Genes Dev. 12:2175-2187). Proteins encoded by AUX1, PIN1 and PIN2 genes which have been identified to be important for auxin transport and are putative membrane proteins that have significant homology with a number of bacterial membrane transporters (Luschnig, C. et al. supra; Gälweiler L. et al., (1998) Science 282:2226-2230; Bennett, M. J. et al., (1996) Science 273:948-950; WO 99/63092-A1; U.S. Application Ser. No. 60/087,789; EP 0 814 161 A1), consistent with a role for these proteins in auxin transport.
Since auxin affects several aspects of plant development, and polar transport is a vital component of auxin function, it is envisioned that proteins involved in auxin polar transport may serve as potential targets for new herbicide discovery and design. Blocking of normal function of these auxin transport proteins can cause severe plant growth defects; this is supported by the phenotype of mutants where a particular auxin transport protein has been rendered nonfunctional, particularly the Arabidopsis pin1 mutants. In addition, since some of these auxin transport proteins have been shown to be root-specific and impact root development to a significant degree, manipulation of auxin transport proteins may be a powerful strategy for developing more robust root systems in plants, which in turn may enhance food production, especially in arid climates.